1. Field of the Invention
This invention relates to devices and methods for attenuating energy that is transmitted underwater.
2. General Background
Many underwater engineering projects generate significant amounts of sound and other energy. This energy can have adverse consequences on marine ecology. For instance, the energy generated by a pile driving hammer can be great enough to kill fish that swim nearby. Especially when such noisy underwater projects are undertaken in environmentally sensitive areas, these ecological consequences are unacceptable.
A number of techniques have been developed to mitigate the adverse biological consequences of underwater construction. The first technique is to stage the project so that noisy phases occur only at times when the biological consequences are minimal. For instance, if the project is in a waterway traveled by anadromous or catadromous fish, noisy phases can be postponed when the fish are migrating. However, this technique is far from ideal, both because it is wasteful to allow labor and equipment to sit idle waiting for fish to migrate, and because most waterways have a residual fish population at all times.
The second technique is to erect a cofferdam around the project. The cofferdam can be constructed using traditional methods such as sheet piling, or by less traditional methods. For instance, an oversized casing tube can be fitted over a pile casing that is being driven, and then the water can be evacuated from the area between the casings, either partially by injecting air bubbles or fully by dewatering the annular space. The air within the casing or other cofferdam does attenuate the energy from the construction project, but this technique is quite expensive. Indeed, for some underwater projects, it is cost prohibitive to establish a persistent envelope of air around the work area.
A third technique is to enshroud the underwater construction area with a stream of bubbles. Like a cofferdam, this technique uses air to attenuate the energy, but unlike a cofferdam very little structure is needed. Indeed, this technique only requires bubble-producing units to be placed around and at the bottom of the construction project. The bubbles then travel from the bubble-producing units to the surface, blanketing the project in sound-dampening air.
While elegant, this technique is ineffective in areas of deep water or strong currents. In these circumstances, the bubbles disperse too far laterally while traveling upward, and cannot completely envelop the project. To contain the bubbles as they ascend, a skirt or blanket of flexible material can be placed around the work area. However, this technique can also be expensive, and is not particularly robust, since the flexible material can be torn or damaged. Also, the flexible material acts like a sail, and therefore this system is not appropriate for areas of high current. A substantial support frame would also be required to implement this system.
Thus, there is a need for a system that can robustly and inexpensively create a curtain of bubbles around underwater construction sites, even in areas of deep water or strong current.